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Bridge load rating question

Bridge load rating question

Bridge load rating question

(OP)
Need some advice from any bridge engineers.  Reviewing a bridge load rating report for a single span multi girder bridge for the std AASHTO truck loads.  Bridge span is 26 ft and the width is 17 ft, single lane.  Bridge consists of five W18x50 beams spaced at 3.96 ft, simple span.  Deck consists of 8" th precast concrete deck planks.  The ends of each plank are connected to only the exterior beams with shear studs.  The planks only bear on the interior beams, there is no connection.   There is no bracing from beam to beam, so the unbraced length of the interior beams is 26 ft, and they are the controlling member.  The rating was performed using both the AASHTO Load Factor Design and Allowable Stress Design methods from the Std Specs for Highway Bridges manual.  The report results using the LFD method give moment inventory and operating ratios of 0.4 to 0.7, so the bridge would have to be posted for lower loads.  .  The report results using the ASD method give moment  inventory and operating ratios of 1.4 to 2.0, so the bridge would not have to be posted.  The report summary concludes by ignoring the LFD results and using the ASD results, stating that the bridge does not need posting and has a capacity greater than the std AASHTO truck loading.  There is no explanation on why engineer chose to ignore the LFD rating results.  
The AASHTO ASD method seems very simplistic and does not address unbraced length with any degree of complexity.  The LFD method seems to be a more extensive analysis method.  The calcs appear to be correct and follow the AASHTO methods.  Am I missing something here?  Is one method better than the other?  I am not experienced with bridge design and analysis.  It makes no sense to me to have very divergent results, and then just go with the favorable result.  Seems like poor engineering judgement at a minimum.  Also, I did a capacity analysis of the bridge beams using the AISC  ASD manual, and the results are similar to the AASHTO LFD results.  Any suggestions?
 

RE: Bridge load rating question

What is your max. moment??

RE: Bridge load rating question

Both methods should provide very similar results, as the major difference in between LFD and ASD is application of the variable safety factors for different loadings versus 1.82 SF for allowable stress method.
So the rating should be almost the same for 40' spans, and lower for LFD for shorter spans, and higher for ASD for longer spans, where the DL participation w/ SF of 1.3 is smaller. In short, it's just different accounting philosophy.
In your case, the mistake is your assumption that the unbraced length is 26'. Buckling of the top compression flange involves rotation of the beam, or part of beam, which will be prevented by the planks bearing on the top flanges, so your beams are at list partially braced, as the flange under loaded plank will not rotate.     
 

RE: Bridge load rating question

(OP)
104 ftk from the HS20 LL, and 37.7 ftk from the DL, not factored.  

RE: Bridge load rating question

(OP)
wiktor, is that std practice to consider a steel beam as least partially braced by a concrete deck bearing on the top flange with no physical connection?  seems by that logic no bridge of this type would need bracing, which does not seem to be what you see in practice.   

RE: Bridge load rating question

I am confused, how can you have shear studs connected to precast plank?  

RE: Bridge load rating question

DWHA - these shear studs can easily be accounted for by grout pockets that are open when placing the planks and then grouted for composite action.  Recall that this is only on the exterior W sections.

It is not common practice to assume the WF beams as partially braced.  If a large enough load were to cross the bridge the beams would simply rotate - no stopping that - short of mechanically fastening them.

While the methods should give similar results, they will differ.  The newer 2008 Manual for Condition Evaluation of Bridges can offer some insight to this and should be consulted.  

ASD is always thought to be the most conservative of methods available and that alone may have played a role in the originator's decision.  

As a reviewer, it is well within your role to ask the originator to provide additional back-up or to explain his decision thoroughly so that you and your client understand it.  There may very well be a mistake.

 

Regards,
Qshake
pipe
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
 

RE: Bridge load rating question

Yes, it will be a standard practice to consider a beam to be partially braced by a bearing deck, even without physical attachment of the planks to the flanges.
This type of construction is commonly used in temporary or make up bridges where only outside beams are attached to the deck.
    

RE: Bridge load rating question

Your moment of 104 k-ft seems low. Can you describe how thats obtained.

RE: Bridge load rating question

bluetree

I've run some quick and dirty numbers on your bridge, and it appears that the calculated ratings are correct.  It is unusual for this magnitude of discrepancy between ASD and LFD, but in this case, the result is due to the assumed unsupported length.  The rating under LFD is governed by Eq 10-103c, which has a similar format to the allowable stress shown in Table 10.32.1A.

Article 10.48.4 reads: Members not meeting the lateral bracing requirements of Article 10.48.2(c) shall be braced at discrete locations...Bracing shall be provided such that lateral deflection of the compression flange is restrained and the entire section is restrained against twisting.  I don't believe precast deck on steel beams meets this definition of Bracing.

If this bridge were originally designed under ASD rules, then some DOTs will allow load rating under ASD.  That may be why the designer took this route.  However, seeing the magnitude of the discrepancy, I agree with you that it was a bad choice.  If I were you, I would not sign off on this.

RE: Bridge load rating question

(OP)
thanks for everyones input.  I found the reason for the discrepancy between the LFD and the ASD results.  I guess I will describe it for anyone who may be interested.  The ASD equation in Table 10.32.1A gives an allowable bending stress using Sxc, section modulus with respect to the compression flange, which is about 37 in3.  In the report, when they calculated the inv and operating ratios they calculated the DL and LL bending stresses by dividing the DL and LL moments by the full Sx of the W18 beam, 89 in3.  As a result, the DL and LL bending stresses are lower than they should be, resulting in inv and operating ratios well over 1.0. If you correct the DL and LL bending stresses using the Sxc value, you get ratios that match the LFD results.   

RE: Bridge load rating question

I have to admit, I would have used Sxc=89in3.  How do you get Sxc=37 in3?  Where does AASHTO show how to do this?

RE: Bridge load rating question

miecz,

I agree with you. Sounds like a grocery store math mix up. Whats worse is that there is no W18 bracing. Signing off on this design would be a huge mistake.

RE: Bridge load rating question

(OP)
I did not find in AASHTO how to calculate Sxc, it was done in the report.  I have not come across Sxc before, but I do not use AASHTO much. It was calcuted in the report as below, and seems logical to me.  I assume the AASHTO ASD method uses this lower value Sxc to account for lower beam capacity with large unbraced lengths.  

Sxc = (1/12*b*d^3 + a*D^2) / D

b = 7.5" W18 fl width
d = 0.57" W18 fl th
a = 7.5 x 0.57
D = 9" - 0.57"/2  distance from fl centroid to W18 centroid

RE: Bridge load rating question

I have to join in on the unbraced length disagreement. The requirement is to prevent displacement or rotation. If a rigid deck is crossing it, it cannot rotate without vertical deflection to clear the underside of the deck. it must go through a higher stored energy of the vertically deflected shape to get to the lower stored energy of the buckled shape, it ain't gonna do it.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Bridge load rating question

paddingtongreen

Take a look at the LFD inventory rating of 0.4. That is the design level. Not good. If it was a slab that was poured I would agree but not precast planks.

RE: Bridge load rating question

That Sxc does not does not have an unbraced length component. It makes no sense to arbitrarily reduce it for all cases except continuously braced beams. The reduction should vary in some proportion with the unbraced length.


As an aside, we once looked at the frictional resistance to lateral beam deflection as a function of the load. By the time the unbraced length became critical, there was enough friction to resist with a good safety factor.

@cap4000, I trust precast, usually pre-tensioned, more than CIP, because there is better QC.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Bridge load rating question

I have seen testing done for poured slabs but not precast. The problem comes about when a heavy truck load deflects the beam and separates the flange from the slab. The friction must be strong enough to transfer horizontal shear stresses to brace the beam. I just don't see that working for a precast slab bridge designed for 75 to 100 years.

The Sxc looks somewhat like a composite slab design whereby the c represents the compression component of the slab and the entire steel beam is in tension or the Sxt. Just a guess about the section modulus values.

RE: Bridge load rating question

I see that miecz quotes "Bracing shall be provided such that lateral deflection of the compression flange is restrained and the entire section is restrained against twisting." so I'm shot down.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Bridge load rating question

Cap4000 makes excellent points.

bluetree-

The formula for Sxc doesn't make sense to me.  The numerator adds two components.  The 1/12*b*d^3 term is the contribution to the moment of inertia of the flange about the vertical axis, while the a*D^2 term is the contribution of the flange to the moment of inertia about the horizontal axis.

paddingtongreen-

On AASHTO's definition of lateral bracing.  I'm merely quoting AASHTO 10.48.4, which requires both.  Not saying I agree with that, but if your have to meet AASHTO, DOT's tend to want you to follow the letter of the code.
 

RE: Bridge load rating question

Bluetree,
For all means the interior beams shall be considered unbraced. I followed the formula for allowable stress in the table 10.32.1A of the code.
Fb = (50000/Sxc)(Iyc/L)*sqrt(0.772*J/Iyc + 9.87(d/l)^2)
for W18x50
Fb = (50000/ 89)*(20.0/ 312)*SQRT(0.772* 1.24/20.0+9.87*( 18/ 312)^2) = 10.25 ksi  
Moment capacity M = 89(10.25)= 912 kip-in = 76 kip-ft
Rating factor RF = (76-37.7)/104.4 = 0.37
RF is lower than calculated by LFD, which is expected for ASD.
 

RE: Bridge load rating question

yakpol is right.  My quick and dirty calculations were a little too quick.

RE: Bridge load rating question

(OP)
miecz
The formula for Sxc is based on the parallel axis theorems for moments of inertia.  The term 1/12*b*d^3 is the moment of inertia of the flange about its own centroidal horizontal axis.  The term a*D^2  is the moment of inertia of the flange area about horizontal axis of the W18 section.  

RE: Bridge load rating question

ASD IR H 28.0 & HS 27.1
    OR H 42.9 & HS 40.1

LFD IR H 21.8 & HS 20.4
    OR H 36.4 & HS 34.0

I did not know how old your bridge is. I assume fy=36ksi. If your bridge is between 1937 to 1963 then fy=33ksi.
If your bridge is between 1905 to 1936 then fy=30ksi.
 In regards to bracing of the deck, read "Bracing Effects of Bridge Decks" Report No. 1239-4F, Center for Transportation Research, University of Texas at Austin, Otober 1993.

In Texas, your bridge would not require load posting. I do hundreds of load rating calculations a year.

RE: Bridge load rating question

ASD IR H 28.0 & HS 27.1
    OR H 42.9 & HS 40.1

LFD IR H 21.8 & HS 20.4
    OR H 36.4 & HS 34.0

I did not know how old your bridge is. I assume fy=36ksi. If your bridge is between 1937 to 1963 then fy=33ksi.
If your bridge is between 1905 to 1936 then fy=30ksi.
 In regards to bracing of the deck, read "Bracing Effects of Bridge Decks" Report No. 1239-4F, Center for Transportation Research, University of Texas at Austin, Otober 1993.

In Texas, your bridge would not require load posting. I do hundreds of load rating calculations a year.

RE: Bridge load rating question

There was some research done that if you had large dead loads on the top flange, the top flange would not rotate.The weight of the 8" slab provides frictional resistance to movement of the top flange. See report referenced in my previous posting. Looking at the numbers they gave you, I suspect there might be an error in LFD calculations.

RE: Bridge load rating question

(OP)
TXBRIDG,
The bridge was built in 2001 and the steel is 50 ksi. The concrete deck is not monolithic, it consists of seven 4 ft wide precast deck planks. Scanned the report you mention and had some questions. So the design practice of the beam being considered braced at the wheel load point for short span steel bridges of my type, is this standard practice in a few, some, all state DOT's?  Is this beam bracing under the wheel load accepted by AASHTO and is it stated in the AASHTO std specs for hwyway bridges manual?  So for a bridge of my type, the beam unbraced length would constantly change as say the last HS20 truck axle load rolled across the bridge.  And lastly, what value for Sxc did you use in your IR and OR calcs in the above thread?   

RE: Bridge load rating question

No, it is not stated in AASHTO manual, When doing load ratings on existing bridges, some engineering judgment is used. TxDOT has a bridge load rating manual that allows the top flange to be fully braced under certain condition such as the timber deck planks covered with over 6" of base and asphalt material. I ran your bridges above with two TxDOT programs. I used 36ksi for both programs. The ASD program used Sx=88.9 and the LFD program used Zx=101 since beam meet the conditions of AASHTO 10.48.1. If the beam did not meet the conditions of AASHTO 10.48.1 then the beam would have fall under AASHTO 10.48.2 and the LFD program would have used Sx=88.9
Research has shown that the beams does not have to be embedded in the deck nor does the deck have to be concrete to provide effective lateral bracing. The first 3 cases lised below may be analyzed as fully braced. In the last two cases the beam should be analyzed with an unbraced length equal to half the span length. In the last case, if the depth of the base is substantially greater tahn 6 inches, the beam may be analyzed as fully braced.
1. Concrete deck cast on top of the compression flange
2. Corrugated metal deck. The deck sections should be interconnected adequately to insure that the deck acts as a unit.
3. Lmainated timber deck
4. Timber planks deck with timber runner. Each of the two runners should be 2 feet wide and the individual runners planks should be securely attached to the deck.
5.Timber plank deck covered with at least about 6 inchges of base and asphalt overlay.

I am not sure if this is used in other DOT's.

The weight of your 8" slab provides enough dead load to provide frictional resistance to keep the top flange from moving laterally so it is considered fully braced regardless if the load is H 20 or HS 20.

RE: Bridge load rating question

I have the 1960 Lateral Beam Bracing article by George Winter. He concludes for a 30 ft span and a uniform load of 2k/ft friction alone would in fact brace the beam. However he does NOT recommend using friction as a bracing mechanism.  

RE: Bridge load rating question

TxDOT has done extensive research in the effects of lateral bracing and the items listed above in my previous posting is straight from their bridge load rating manual based on their current research. You still have to look at the structure and use your engineering judgment to what you feel comfortable with. I have done thousands of load ratings and I know what I feel comfortable doing based on my 20 years of experience. I once had a boss long time ago that would say this when he was uncomfortable with something his advise was "When in doubt reinforce through out".

RE: Bridge load rating question

Bluetree,
If you are with a city or county official, have your State DOT review the load rating. By the description of the bridge, I assume it is an off-system bridge (bridge owned by county or city or some other local government).State DOT's are given the responsibility to make sure that off-system bridges are inspected and load rate by the federal government.

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